R. William Field, PhD
Humberto Choi, MD
The Environmental Protection Agency (EPA) has concluded that about 21,000 people die each year of lung cancer related to radon gas exposure, making it the second leading cause of lung cancer death in the United States. Although the EPA and various other organizations, including the National Radon Safety Board, have recommended wide-scale radon screening in domestic residences at risk for exposure, national efforts to reduce radon concentrations, including programs that promote radon-resistant new home construction, have been curtailed by budgetary cuts.
However, according to R. William Field, PhD, Professor of Epidemiology at the University of Iowa College of Public Health, Iowa City, protracted exposure to radon decay products is the leading environmental cause of cancer mortality in the United States. In a recent interview with The ASCO Post, Dr. Field explained more about exposure to radon: “Radon is a naturally occurring radioactive gas. The magnitude of the radon concentration indoors is directly related to the amount of radon produced in the underlying soil and bedrock, the soil’s permeability, and the building’s construction. The air pressure inside homes is slightly lower than in the ground, which draws radon gas into the basement through cracks and other penetrations in the building’s foundation.”
Asked about at-risk populations, Dr. Field said: “There’s a known risk of lung cancer associated with the synergistic relationship of smoking and radon exposure. According to published data, an individual’s genetic makeup also plays a role in increased risk. For example, about half of the population are missing a gene called GSTM1, which modulates the carcinogenic pathway following radon exposure, making individuals missing GSTM1 more susceptible to radon exposure–related lung cancer.”
A Brief History
Radon is a colorless, odorless, and tasteless radioactive gas that is formed by the natural decay of uranium-238, and subsequently radium-226, in rock, soil, and water, which occurs at different levels in all 50 states. The threat of the radon hazard was first documented in 1879, when scientists described a high mortality from respiratory diseases in radium miners in the Erz Mountains in Eastern Europe. However, the connection between radon and cancer was not made until 1929, when formal, published autopsy reports found that more than 50% of the miners of the Erz Mountains died of lung cancer.
We know the main route of radon exposure is by inhalation. Since radon is biologically inert, it is readily exhaled after it reaches the lungs.— Humberto Choi, MD
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Scientists in the United States first recognized the threat of radon exposure in the 1970s, when epidemiologists found that uranium miners were developing lung cancer at higher-than-average rates; the tunnels they worked in contained high concentratrions of radon gas. However, in 1984, an engineer at the Limerick Generating Station (a nuclear power plant in Pennsylvania) set off radiation alarms as he reported to work. It took a while to solve this conundrum, but public health officials finally inspected the engineer’s home and discovered high levels of radon gas, especially in the underground basement and recreation area.
The incident was reported in the media, gaining public attention that spurred action by the EPA and the U.S. Centers for Disease Control and Prevention. By 1988, the International Agency for Research on Cancer classified radon as a known human carcinogen. Shortly thereafter, Congress passed the Indoor Radon Abatement Act, which funded state and federal radon surveys of schools and federal buildings. The Act’s long-term goal was to reduce indoor radon levels to that of outdoor levels, an effort that has stalled over the ensuing decades.
More Research Needed
Dr. Field stressed that the early body of evidence of radon’s carcinogenicity came from 11 large epidemiologic studies of miners exposed to radon gas and from the 1999 National Research Council’s pooled analyses of those studies. He also pointed out there have been similar pooled analyses of residential radon studies performed in China, Europe, and North America, which have clearly demonstrated that long-term exposure to radon, even below the EPA’s action level (4 pCi/L [150 Bq/m3]), increases an individuals’s risk of developing lung cancer.
To elucidate radon’s carcinogenic process, The ASCO Post spoke with Humberto Choi, MD, a pulmonologist at the Cleveland Clinic. “There are multiple hypotheses on the way radioactive radon gas triggers oncogenesis,” Dr. Choi commented. “We know the main route of radon exposure is by inhalation. Since radon is biologically inert, it is readily exhaled after it reaches the lungs. However, radon’s progenies can also be inhaled, either as free particles or attached to airborne particles, such as dust, which they tend to attract as a result of their charged state. The radiation emissions damage the genetic material of cells lining the airways, with the potential to result in lung cancer if the repair process is incomplete,” said Dr. Choi. He noted that alterations in several specific genes have been implicated in lung cancer resulting from indoor exposure to radon: EGFR, TP53, NK2 homeobox 1 (NKX2.1), phosphatase and tensin homolog (PTEN), among others.
Asked about mitigation strategies, Dr. Choi said: “A variety of mitigation strategies have been used, with different degrees of success, but the optimal effort to reduce radon contamination depends on the source or cause, the construction itself, and the features of the soil and climate. That said, there is no clear safe level of radon exposure set in the literature, which makes the debate more problematic. In fact, the Trump administration has proposed completely eliminating the EPA’s radon evaluation and remediation program, which I think is not a good idea. We know that radon gas poses a public health risk. We just don’t know enough, and that will require more research.”
Given that funding for radon control programs in the United States has declined markedly since 1997, and that the number of homes in the United States exceeding the EPA’s radon action level remains historically high, many in the epidemiology community believe there is a need to enhance the EPA’s Radon Program and increase grants to states to educate the public about the risks posed by radon. According to Dr. Field also stressed the importance of the U.S. Preventive Services Task Force’s low-dose computed tomography (CT) screening to identify early stage lung cancer in individuals with a heavy smoking history. He noted that the program provides a “teachable moment” to educate individuals who call the screening program about other ways to reduce the risk for lung cancer, such as testing for radon and—if indicated—taking action to reduce radon concentrations.
Moreover, the National Comprehensive Cancer Network Clinical Practice Guidelines in Oncology recommend low-dose CT screening for individuals with a 20-pack/year smoking history and documented radon exposure, noted Dr. Field. “The take-home message is that in comparison to all the environmental carcinogens strictly regulated by the EPA, protracted radon exposure in the home, which is not regulated, is a leading environmental cause of cancer mortality in the United States. The pooled residential radon studies clearly demonstrate an increased risk of lung cancer even below the EPA’s radon action level. Fortunately, well-designed remediation systems and homes built radon-resistant significantly lower the risk of radon gas–induced lung cancer,” said Dr. Field.
For more information on radon and the role of health-care practitioners in reducing the burden of radon-related lung cancer, see Reducing the Risk From Radon: Information and Interventions, A Guide for Health Care Providers.
DISCLOSURE: Drs. Field and Choi reported no conflicts of interest.